We have investigated flow to the border area between ischemic and normal myocardium using radioactive microspheres and a coronary perfusion model and have found that collateral flow to ischemic tissue increases as the edge of the ischemic area is approached. The lateral zone of increased collateral flow is substantial (about 15 mm wide) and is present in both endocardial and epicardial layers (wider in epicardium). The distribution of collateral flow after acute coronary artery occulsion agrees with the distribution of necrosis 2 days later. We have found a substantial rim of non-infarcted myocardium epicardially and laterally within the zone at risk defined as the volume of distribution of the occluded coronary artery. We have used a conscious dog infarct model to show that without treatment about a third of the risk area undergoes necrosis; with nitroglycerin treatment infarct size is halved, while after indomethacin the infarct is approximately double in size.